Fig. 1. 



Consider now the ionization-chamber represented in fig. 

 1. The two ends are closed by 

 plates, of which .4 and A ' are 

 alike; so also are B and B'. The 

 substance of .4 and .4 ' is different 

 to that of B and B'. Tlie nature 

 of the side walls is immaterial. A 

 pencil of y rays passes along the 

 axis of the chamber, which is re- 

 presented by a dotted line. The 

 ionization current within the 

 chamber is measured as usual by 

 inserting a high potential electrode 

 connected to an electroscope. 



When the plates .4 and B are inverted, there is a change 

 in the amount of the current; so also when A' and B' are 

 inverted. By an extension of the principle already stated, it 

 ought not to be possible, on the ether-pulse theory, to dis- 

 cover which way the rays are going, up or down in the fig- 

 ure, by comparing the consequence of inverting A and B 

 with that of inverting A' and B'. 



As a matter of fact, the direction can be discovered with 

 ease ; the more easily the greater the difference between the 

 atomic weights of A and B. 



For example, in one experiment of ours the chamber 

 was of cylindrical form, 3 inches 

 high and 10 inches diameter. g 



The plates used were of alumin- 

 ium and lead. The thickness of 

 each plate was a little less than ^^^^^ 



2 mm. Inversion of the top plates M Pi- ^g ^^3 ... ^ 



A and B made a difference in , ^liiir: . 



favour of Al of less than 1 per I J 



cent., i.e., the current was slightly | | 



larger when Al was next the 



:..v? , 



chamber. On the other hand. Fig. 2. 



inversion of the bottom plates 



made a difference of 44 per cent, in favour of Pb, i.e., the 

 current was 44 per cent, larger when the Pb was on top. 

 The details are shown in the figure. Allowance was made 

 for all radiation other than that which proceeded down the 

 conical opening in the lead block. 



It may be wtell to point out that this effect cannot be 

 ascribed to any complication due to secondary or tertiary 

 rays. No doubt the radiation in the chamber is very com- 

 plex ; but the fact is immaterial. Provided that the cham- 

 ber is symmetrical in the first place, then the secondaries 



